imports: Fix relative module path hash-table key

Using ->str was wrong; it fails for relative module paths
like (submod (lib "scribble/blueboxes.rkt") #%contract-defs).

The resulting error could be confusingly attributed to user code, for
example with racket-xp-mode reporting a error in our own
scribble.rkt, with a locus in ->str in imports.rkt.

racket/imports.rkt

@@ -41,7 +41,7 @@
   (->* (syntax?) hash?)
   (define ht (make-hash))
   (define (update! rmp f)
-    (hash-update! ht (->str rmp) f (set)))
+    (hash-update! ht (~a (syntax->datum rmp)) f (set)))
   (define (add! rmp v)
     (update! rmp (λ (s) (set-add s (->str v)))))
   (define (remove! rmp v)